The present invention relates to an actuator for a hydraulic brake, more particularly, such an actuator having a self-adjusting feature to compensate for brake lining wear.
Drum brakes are, of course, well known in the art and typically comprise a pair of brake shoes mounted on a stationary backing plate with a drum rotatably surrounding the brake shoes. The shoes have friction material mounted on their outer surface and means are provided to move the brake shoes radially outwardly until the friction material contacts the interior surface of the brake drum. In a hydraulically actuated system, hydraulically actuated pistons are utilized to move the brake shoes radially outwardly. Return springs are utilized to withdraw the brake shoes from contact with the brake drum.
In large heavy-duty trucks and off-road vehicles, the brake shoes may be attached to a brake spider casting instead of the backing plate. The brake spider casting may define actuating piston cylinders on diametrically opposite sides which contain the brake shoe actuating pistons and plungers. In known systems the pistons are actuated by a hydraulically movable wedge to move the brake shoes radially outwardly into contact with the brake drum. A plunger assembly interposed between the brake shoes and the actuating pistons provides means to adjust the positions of the brake shoes so as to compensate for brake lining wear.
Quite obviously, as the thicknesses of the brake linings decrease, adjustment means must be provided to adjust the relative positions between the brake shoes and the actuating pistons. Typically, known systems have utilized a mechanical system involving threaded plungers interposed between the actuating pistons and the brake shoes to provide the necessary adjustment. However, these devices have not proven totally satisfactory in reliably adjusting the brake linings relative to the brake shoes in order to maintain the proper running clearance. When used on such heavy-duty equipment, the brakes are, of necessity, subjected to rather abusive operating conditions. The purely mechanical nature of the prior art adjusting systems are all subject to corrosion, increased friction in the working components, etc. All of these factors contribute to the limited success of such prior art systems in achieving a reliable brake adjustment system.